Color television picture tube apparatus

ABSTRACT

A color television picture tube apparatus which displaces the electron beam dispersing position in the plane of deflection center of the delta gun shadow mask type color cathode-ray tube in which three electron guns are positioned respectively at the vertices of an equilateral triangle so that the beam spot from each electron gun is incident upon each vertex of the equilateral triangle on the screen of the cathode-ray tbe.

United States Patent [191 Ikeuchi [4 1 Oct. 2, 1973 [54] COLOR TELEVISION PICTURE TUBE 3,430,099 2/1969 Ashley 335/213 UX APPARATUS 3,440,483 4/1969 Kaashoek et a1. 315/24 Inventor: Hiroshi Ikeuchi, Yokohama, Japan Denki Onkyo Co., Ltd., Tokyo, Japan Aug. 31, 1971 Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Sept. 9, 1970 Japan 45/79095 U.S. Cl. 335/213, 313/77 Int. Cl. 1101f 5/00 Field of Search 335/210, 213;

References Cited UNITED STATES PATENTS 2/1972 Chiodi 335/213 Primary ExaminerGeorge Harris Attorney-James E. Armstrong et al.

[57] ABSTRACT 7 Claims, 10 Drawing Figures Patented Oct. 2, 1973 FIGJ FIG.2

3 Sheets-Sheet 1 Patented Oct. 2, 1973 3,763,452

3 Sheets-Sheet f3 FIGA Patented Oct. 2, 1973 3 Sheets-Sheet :5

COLOR TELEVISION PICTURE TUBE APPARATUS BACKGROUND OF THE INVENTION The present invention relates to a color television picture tube apparatus employing the so-called delta gun shadow mask type color cathode-ray tube which is provided with three electron guns positioned respectively at the vertices of an equilateral triangle.

To obtain a fine color picture on the screen of the cathode-ray tube of this type, the following requirements should be satisfied that the electron beam is emitted from each electron gun onto the center of each corresponding phosphor dot on the screen of the cathode-ray tube, the purity of colors is high and the electron beams are converged onto a group of phosphor dots.

These requirements can be comparatively easily satisfied in the case of the cathode-ray tube with a spherically formed screen, whereas it is difficult to satisfy the requirements in the case of a narrow-necked, wideangle deflection cathode-ray tube.

The wide angle deflection cathode-ray tube is advantageous in practical use because the distance between the electron guns and the screen is small and the screen is almost flat with a curvature approximate to that of a flat surface; however, it is necessary to control the electron beams so that the electron beams may be emitted exactly onto the 3-color phosphor dots on the screen because the incident angle and distance of the electron beams which reach the phosphor dots on the screen have the values proper to respective phospher dots.

As known from the above, deviation of color results from the purity of colors and emission of the beam trio onto a group of phosphor dots depends on convergence of electron beams.

The following describes the purity of color and convergence of electron beams in the color television picture tube.

This type of the color television picture tube is comprised, as shown in FIG. 6, of vacuum tubular body 10 consisting of narrow neck portion 11 and conical portion 12 which is conically expanded from the narrow neck portion, three electron guns 20, 30 and 40 which are arranged in parallel so that the electron guns are positioned at the vertices of a equilateral triangle inside said neck 11 and are slightly slanted so that electron beams 21, 31 and 41 intersect at point P on cathoderay tube axis A, that is, at a small hole provided in the shadow mask, deflecting means 50, for example, deflection yoke which deflects vertically and horizontally electron beams 21, 31 and 41 emitted from said three electron guns toward conical portion 12, screen 60 on the internal surface of which a number of phosphor dots, curved plate 70 called the shadow mask which is provided inside said screen and provided with a number of small holes 7,and convergence yoke 80 which controls intervals between three electron beams.

In the case of the cathode-ray tube, with such construction as above, electron beams 21, 31 and 41 are emitted onto screen 60 with phosphor dots at an incident angle as if the electron beams deflect at deflection center 52 of deflection field 51 in reference to cathoderay tube axis A.

There are provided on the internal surface of screen 60 many phosphor dots each of which contains three fluorescent points which glow respectively in three primary colors, blue, red and green.

The phosphor dots are formed by the method shown below in the course of manufacturing the cathode-ray tubes.

The position of deflection center 52 is predetermined and shadow mask is provided. Then, three light fluxes f f and f from spot light sources L L and L arranged at three positions on deflection center 52 are directed to the screen. These three light fluxes are irradiated onto the screen through small holes 71 of the shadow mask while being deflected by lense 90 which is made to meet deflection tendency of the electron beams due to the fleld distribution characteristic of the deflection yoke. The fluorescent material is photographically developed at every three exposure position to the incident light fluxes through the small holes, thus forming a group of phosphor dots for each small hole.

Accordingly, if three electron beams 21, 31 and 41 are deflected at deflection center 52 and are coverged at small holes 71 of the shadow mask to the screen, all electron beams reach the centers of corresponding phosphor dots.

However, a group of electron beams are refracted at deflection center 52 and converged at plane S shown with a dotted line; therefore, if the curvature of focusing plane S is larger than that of shadow mask 70 as in the case of the wide angle deflection cathode-ray tube, a group of electron beams intersect before the small holes of the shadow mask and are inevitably irradiated onto the screen through different small holes.

In this case, the color purity is satisfactory because each electron beams reaches each corresponding phosphor dot but the convergence of electron beams are unsatisfactory.

Therefore, the conventional apparatus is provided with convergence yoke so that a group of electron beams emitted from the electron guns are converged at the same small hole by displacing the electron beams before deflection field 51.

Field 81 of this convergence yoke is provided to vary the interval among electron beams in order to concentrate electron beams 21, 31 and 41 onto a group of phosphor dots on the screen; accordingly, incident angle 0 of electron beam to the small hole of the shadow mask in reference to deflection center 52 is different from incident angle 0 of the light which is re fracted by lens as illustrated in FIG. 8.

Since the electron beams from three electron guns pass the small holes of the shadow mask through the incident paths different from those through which the light from the spot light source passes the small holes of the shadow mask, the spot of each electron beam deviates from the center of each of corresponding phosphor dots. In this case, the purity of colors is unsatisfactory even though the convergence of electron beam is satisfactory.

The following describes the relations between said purity and convergence.

If the electron beams are deflected at deflection center 52, it is duly known, from the fact that the fluorescent screen is made by the photographical technique, that the optimum purity of colors can be obtained at any deflection angle.

This means that each electron beams deviates from the center of each corresponding phosphor dot if the electron beams are deflected in the compensating plane other than the deflection center.

Accordingly, if the convergence is compensated at a position near the electron guns from the deflection center, the electron beam or beams, deviate from the center of phosphor dot or dots. If the amount of correction is large, the electron beam also deviate from the phosphor dot center to cause mislanding and the purity will deteriorate.

Deterioration of the purity due to correction of convergence can be prevented by correcting convergence at the center of deflection. Thus, it is said impossible to correct the convergence in the main deflection circuit.

The present invention provides a color television picture tube apparatus capable of effectively preventing deterioration of the purity of colors.

SUMMARY The present invention is comprised of the delta gun shadow mask type color cathode-ray tube in which three electron guns are arranged at the verteces of an equilateral triangle with l angular intervals, a deflection means such as, for example, a deflection yoke which deflects and scans a group of electron beams so that the positions of the spots of three electron beams on the screen, when the electron beams are not converged, conform to the vertices of an equilateral triangle on at least one certain line including the center of the screen and so that said positions conform to the vertices of the isosceles triangles in the same shape at the symmetrical positions on other lines including the center of the screen in reference to the center of the screen, a correcting field generating means which deflects the electron beams so that the spots of electron beams conform respectively to the vertices of an equilateral triangle on the screen by shifting at least two electron beams in the horizontal direction of three electron beams arranged respectively at the vertices of said isosceles triangle or so that the distances between the centers of convergence and spots of electron beams are equal, and a convergence yoke which converges said electron beam trio so that the electron beam trio is converged at each small hole of the shadow mask, wherein the correcting field generating means is positioned at the position where the correcting field is generated at the deflection center of said deflection means and the electron beam trio is deflected under such condition that the equal distance between each spot of electron beam trio which is positionally adjusted and the center of convergence is shorter than the longest of the distances between each spot before positional adjustment and the center of convergence and is longer than the shortest of said distances.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated in detail in the accompanying drawings whereof:

FIGS. 1 and 2 are the front views of the screen on which the relative positions of the spots of the electron beam trio are shown;

FIGS. 3a, 3b and 3c are the explanatory diagrams for the correcting method of the beam spot trio on the screen;

FIG. 4 is a front view of the core of the deflection yoke indicating an embodiment of the correcting field generating means;

FIG. 5 shows an example of the correcting current waveform to be supplied to said correcting field generating means;

FIG. 6 is a cross sectional plan view of a cathode-ray tube indicating the construction of the conventional apparatus;

FIG. 7 is a cross sectional plan view of a cathode-ray tube illustrating the method to form three-color phosphor dots; and

FIG. 8 is a magnified view of the important part of the cathode-ray tube illustrating the incident angle of electron beam trio into the shadow mask.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown the characteristic of the electron beam spot trio appearing on the screen through the deflection yoke of the apparatus according to the present invention.

The beam spot trio is a combination of beam spot 311 directed to a phosphor dot, for example, for blue, beam spot 411 to a phosphor dot, for example, for red and beam spot 211 to a phosphor dot, for example, for green. Each spot is deflected on certain line I including center 0 of the screen, for example on the diagonal line on the screen so that the beam spots conform to the vertices of the equilateral triangle and is deflected on other line 1' including the center of the screen, for example, on the vertical line and horizontal line so that the beam spots conform to the vertices of the isosceles triangle.

The shape of the isosceles triangle is such that the perpendicular bisector is small relatively as compared with the base at the top and bottom parts of the screen and the perpendicular bisector is large relatively as compared with the base at the right-and left-side parts of the screen.

Any of said triangles is inverted in direction because three electron beams interesect at a small hole of the shadow mask. Accordingly, in the case of a cathode-ray tube in which three electron guns are arranged at the vertices of an inverted equilateral triangle, the beam spot trio is dispersed so as to conform to the vertices of an equilateral triangle inverse to the shape of the triangle shown.

The triangle formed by the beam spot trio appearing on the screen will be an equilateral triangle at every position on the screen if deflection field 51 is uniform and the screen has a spherical surface. However, the screen of the television picture tube currently used is almost flat, the deflection coil of the existing deflection yoke is short and the deflection coil is formed in a conical shape along the conical part 12 of the cathode-ray tube. It is therefore impossible to obtain a uniform field and it is possible only to disperse the beam spots in the shape of the equilateral triangle on certain line I on the screen. On other lines on the screen, dispersion of beam spot trio only conforms to the isosceles triangle.

Dispersion of the beam spots onto the vertices of an equilateral triangle is not limited to the diagonal line only on the screen and is possible on a vertical or horizontal line including the center of the screen. In this case, it is satisfactory for the electron spot beams to form the isosceles triangle on lines other than the lines on which an equilateral triangle is formed, for example, on the diagonal line. This is possible.

However, it is most desirable to disperse the electron beams in a shape of equilateral triangle at the corners of the screen where the electron beams are deflected to the most extent as shown in FIG. 1.

If the deflection method as described above is employed, a marked aberration occurs at the end of vertical and horizontal lines I as described above and causes the electron beam spots to disperse onto the vertices of the isosceles triangle. Since convergence of electron beams dispersed in the vertical and horizontal directions is possible by supplying a simple correcting current to the convergence yoke, convergence is easy.

It is possible to converge the electron beam spots which disperse onto the vertices of an equilateral or isosceles triangle on the screen. If the electron beam spots which are dispersed to conform to the vertices of the isosceles triangle are converged, the amount of movement of one or two of three electron beam spots become large and there is possibility of the beam spots deviating from corresponding phosphor dots on the screen, thus deteriorating the purity of color.

FIG. 2 illustrates the characteristic of a group of electron beam spots which are corrected by the correcting field generating means of the apparatus according to the present invention.

This correcting field generating means permits shifting of three electron beams 21, 31 and 41, which are interrelated, at the deflection center of the deflection yoke so that the electron beam spots conforming to the vertices of the isosceles triangle conform to the verteces of the equilateral triangle on screen 70.

Accordingly, the beam spots are dispersed to form an equilateral triangle at each point on the screen as shown; in this case, the equilateral triangles are varying in size.

If the dispersing position of the electron beam spots is corrected so that the beam spots conform to the vertices of the equilateral triangle, the distances between center C of convergence and respective beam spots become equal and the electron beam spots are shifted to the center of the same distance to direct onto the corresponding phosphor dots by converging the beam trio through field 81 of convergence yoke 80.

FIG. 3a and 3c show the relations between the dispersing of the beam spots and the amount of movement of the beam spots due to convergence. In case of a isosceles triangle with small perpendicular bisector h and long base d, it is satisfactory to shift beam spot 311 by distance h and, at the same time, two beam spots 211 and 411 by distance -d' in the direction where these two beam spots approach each other.

In case of an isosceles triangle with large perpendicular bisector h and short base d as shown in FIG. 3b, it

.is satisfactory to shift beam spot 311 by distance h and, at the same time, other beam spots 211 and 411 by d' in the direction of dispersion.

The beam spots for which the dispersing position is determined as described above conform to the verteces of the equilateral triangle. If these beam spots are converged, beam spots 21 l, 311 and 411 move by the same distance and reach corresponding phosphor dots B, R, and G.

Accordingly, the purity of colors is high because any beam spot does not require a shift of long distance.

FIGS. 4 and 5 show a means to generate correcting field M.

Existing core 53 is a part of deflection yoke 50. It is provided with a pair of vertical deflection coils and a pair of horizontal deflection coils which are not shown herein.

Said core 53 is provided with correcting coil for generating the correcting field in addition to the deflection coil. Coil 10th is designed to form correcting field M at the center of deflection field of the deflection yoke.

Coil 100 consists of four coil portions 101, 102, 1193 and 104, two, 1111 and 1112, of which are oppositely arranged on vertical line y in reference to axis A of cathode-ray tube as origin and other two 103 and 104 are oppositely arranged on horizontal line X.

Said coils provide the field distribution which is symmetrical in reference to two axes Z deflected by 45 from axes X and Y which intersect at a right angle, and generate the line of magnetic force, as shown, which deflects beam 31 in the vertical direction along axis y and two beams 41 and 21 in the horizontal direction to an oblique direction having X-axis component. The coils are devised so that, when beam 31 is shifted upwardly with this field, other two beams 21 and 41 are shifted to approach each other at the same time and, when beam 31 is shifted downwardly, other two beams 21. and 41 are shifted to the direction of dispersion.

In FIG. 4, coil 100 is toroidally wound around core 53. In this case, the flat-shaped coil can be used instead of this coil, and flat-shaped coils 101', 102', 103' and 104' are arranged at the positions shown with dotted lines in FIG. 4.

When using correcting coil 11111 as described above, the correcting coil 1110 is mounted together with the deflection coil of deflection yoke to concentrically overlap the deflection center of correcting field M on the deflection center of deflection field 51.

Thus, symmetrical arrangement of four coils as described above is advantageous because electromagnetic induction against the deflection coil of the deflection yoke is substantially offset and interference to the deflection yoke can therefore be avoided.

However, correcting field M can be generated by supplying the correcting current together with the deflection current to the deflection coil of the deflection yoke. In this case, independent correcting coil 100 is not required.

FIG. 5 shows the correcting current which is supplied to said correcting coil 100.

The current flow shown in FIG. 5 is to change dispersion of the beam spot trio as shown in FIG. 1 into dispersion shown in FIG. 2.

The magnitude of correcting current is to be supplied determines the extent of correction; accordingly, the condition that the current value is 0 indicates that no correction is required, the beam spots are dispersed onto the vertices of the equilateral triangle and the beam spots are converged onto the center of the screen. I

The point where the value of correcting current is large coincides with the point where the beam spots are deflected to the center of the top and bottom of the screen and of the right and left-side parts.

The correcting current waveform becomes maximum as shown in waveform I when the beam spots reach the center of the top of the screen. At this time, the direction of the magnetic flux of the correcting field is as shown in FIG. 4; therefore beam 31 in a vertical direc tion is shifted far from center A on axis y and other two beams 21 and 41 are shifted to approach each other.

Since it is necessary to shift similarly the electron beams even when the beam spots are deflected to the center of the bottom of the screen, the correcting current increases again as waveform b at the end of vertical deflection IV.

On the contrary, the direction of the correction current flow is reversed at the central part of the vertical deflection cycle and becomes the maximum.

When such the correction current is supplied, the magnetic flux of correcting field M flows in the direction reverse to that shown in FIG. 4; accordingly, electron beam 31 is deflected to approach the central point and other two electron beams 21 and 41 are deflected in the direction of dispersion. Therefore, the electron beams which are dispersed to form an isosceles triangle with large perpendicular bisector h at the centers of the right and left sides on the screen are deflected to the dispersing position where an equilateral triangle is formed in the correcting field generated with waveforms S and S of the correcting current. K

The waveform of the correcting current should be able to correct dispersion of electron beams in the form of the isosceles triangle on the scanning line into dispersion in the form of the equilateral triangle.

Dispersion of the electron beams shows a proper distribution due to the characteristic of the specified cathode-ray tube and the deflection coil provided thereon; accordingly, it is difficult to determine the general correcting current waveform. If dispersion of electron beam spots is distributed as shown in FlG. 1, triangular wave 01 of horizontal deflection cycle [H is coupled to vertical deflection cycle N with envelop Q2 of the triangular wave.

Actually, in most cases, the waveform of triangular wave Q2 shows a bent side. Furthermore, the sine wave and parabolic wave can be substituted for triangular wave Q1.

According to the present invention, the electron beam trio is positionally corrected at the deflection center so that the electron beam trio which is dispersed to conform to the vertices of the isosceles triangle conform to the vertices of the equilateral triangle. In this case, it is necessary there is relationship between this equilateral triangle and the original isosceles triangle.

The direction where the beam spots are converged is fixed at the times and the center of convergence coincides with the center of the equilateral triangle. Accordingly, the distance between the center of convergence and each beam spot, that is, the convergence distance is equal in case of the equilateral triangle.

On the other hand, if perpendicular bisector h of the isosceles triangle is long, the convergence distance of beam spot 311 in a vertical direction is also long and the convergence distance of other spots become short. if perpendicular bisector h is small, the convergence distance of beam spot 31 l is short and that of other two spots is long. i

The convergence distance of the equilateral triangle resulting from correction should be shorter than the longest convergence distance of the isosceles triangle 7 and longer than the shorter convergence distance of the isosceles triangle.

Only one equilateral triangle which satisfies the above condition is obtained from each isosceles triangle if the ratio of variation of the perpendicular bisector to variation of the base of the isosceles triangle is fixed, and therefore it is possible to fix the ratio of decrease of the long convergence distance of the similar isosceles triangle to the increase of the short convergence distance at all time. This provides an advantage to easily determine the waveform of the correcting current.

As known from the above, beam 31 in the vertical direction vertically moves along axis y to vary the convergence distance. This vertical movement of beam 31 can be obtained by shifting two beams 21 and 41 arranged in the horizontal direction including the amount of vertical movement of beam 31.

These two beams should be shifted slantly downward so as to approach each other and slantly upward so as to disperse.

In this case, the position of beam 31 in the vertical direction is the reference position. Accordingly, the base of the equilateral triangle obtained through correction on the screen is deviated vertically from the base of the original isosceles triangle. The size of the equilateral triangle obtained from this method can be determined to be the same as that obtained when three electron beams are shifted at the same time.

Thus, correction will be easy because the correcting field need not act on electron beam 31.

It is known from the above that deterioration of the purity of colors can be minimized and the electron beam trio can be converged.

What is claimed is:

1. A color television picture tube apparatus comprised of:

a. a delta gun shadow mask type color cathode-ray tube in which three electron guns are arranged to conform respectively to the vertices of an equilateral triangle,

b. a deflecting means which deflects and scans electron beams so that three electron beam spots on the screen when the beams are not converged are respectively positioned at the vertices of an equilateral triangle on at least one certain line including the center of the screen and at the vertices of an isosceles triangle which is symmetrical in reference to the center of the screen on othe lines including the center of the screen.

. a correcting field generating means which makes the convergence distance shorter than the longest convergence distance of the original isosceles triangle and longer than the shortest convergence distance of the original isosceles triangle and provides a uniform convergence distance by deflecting at least two electron beams in the horizontal direction at the deflection center of the deflection means so that the electron beam spots conform respectively to the vertices of the equilateral triangle on the screen, and

d. a convergence yoke which converges a group of electron beams to concentrate the electron beams into the small holes of the shadow mask of the cathode-ray tube.

2. An apparatus according to claim 1 wherein the deflection means deflects the electron beams so that the electron beam spots on the screen are dispersed to conform respectively to the vertices of the equilateral triangle on diagonal lines on the screen, to the vertices of an isosceles triangle with a long base and a short perpendicular bisector on the line including the centers of the top and bottom or on the line including the centers of the right and left sides of the screen and to the vertices of an isosceles triangle with a short base and a long perpendicular bisector on the rest of these lines.

3. An apparatus according to claim 1 wherein the correcting field, which causes one electron beam positioned at a vertex of a triangle to shift along a vertical line including the center of the deflection field and other two beams to shift in a horizontal direction, is formed by the correcting field generating means, and when the electron beam in the vertical direction is shifted upward, other two electron beams are shifted to approach each other and when the electron beam in the vertical direction is shifted downward, other two beams are shifted to disperse.

4. An apparatus according to claim 1 wherein the correcting field, which does not affect one electron beam in the vertical direction conforming to one vertex of a triangle and shifts obliquely other two beams to approach each other and to depart from the electron beam positioned in the vertical direction and also shifts obliquely the two beams to depart from each other and to approach the electron beam positioned in the vertical direction, is formed by the correcting field generating means.

5. In a deflection yoke having a ring-shaped core provided with a pair of vertical deflection coils and a pair of horizontal deflection coils, the improvement comprising a field correcting coil forming a correcting field at the center of the deflection field of the deflection yoke, said field correcting coil consisting of four coil parts arranged at 90 intervals and connected to form a four-pole magnetic field generating area in which mu- 7. A deflection yoke according to claim 5 wherein four flat-shaped coil parts are provided at the ringshaped core and are arranged so that they are positioned on two axes deflected by 45 from the vertical and horizontal axes which intersect at a right angle.

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1. A color television picture tube apparatus comprised of: a. a delta gun shadow mask type color cathode-ray tube in which three electron guns are arranged to conform respectively to the vertices of an equilateral triangle, b. a deflecting means which deflects and scans electron beams so that three electron beam spots on the screen when the beams are not converged are respectively positioned at the vertices of an equilateral triangle on at least one certain line including the center of the screen and at the vertices of an isosceles triangle which is symmetrical in reference to the center of the screen on othe lines including the center of the screen. c. a correcting field generating means which makes the convergence distance shorter than the longest convergence distance of the original isosceles triangle and longer than the shortest convergence distance of the original isosceles triangle and provides a uniform convergence distance by deflecting at least two electron beams in the horizontal direction at the deflection center of the deflection means so that the electron beam spots conform respectively to the vertices of the equilateral triangle on the screen, and d. a convergence yoke which converges a group of electron beams to concentrate the electron beams into the small holes of the shadow mask of the cathode-ray tube.
 2. An apparatus according to claim 1 wherein the deflection means deflects the electron beams so that the electron beam spots on the screen are dispersed to conform respectively to the vertices of the equilateral triangle on diagonal lines on the screen, to the vertices of an isosceles triangle with a long base and a short perpendicular bisector on the line including the centers of the top and bottom or on the line including the centers of the right and left sides of the screen and to the vertices of an isosceles triangle with a short base and a long perpendicular bisector on the rest of these lines.
 3. An apparatus according to claim 1 wherein the correcting field, which causes one electron beam positioned at a vertex of a triangle to shift along a vertical line including the center of the deflection field and other two beams to shift in a horizoNtal direction, is formed by the correcting field generating means, and when the electron beam in the vertical direction is shifted upward, other two electron beams are shifted to approach each other and when the electron beam in the vertical direction is shifted downward, other two beams are shifted to disperse.
 4. An apparatus according to claim 1 wherein the correcting field, which does not affect one electron beam in the vertical direction conforming to one vertex of a triangle and shifts obliquely other two beams to approach each other and to depart from the electron beam positioned in the vertical direction and also shifts obliquely the two beams to depart from each other and to approach the electron beam positioned in the vertical direction, is formed by the correcting field generating means.
 5. In a deflection yoke having a ring-shaped core provided with a pair of vertical deflection coils and a pair of horizontal deflection coils, the improvement comprising a field correcting coil forming a correcting field at the center of the deflection field of the deflection yoke, said field correcting coil consisting of four coil parts arranged at 90* intervals and connected to form a four-pole magnetic field generating area in which mutually opposing poles have the same polarity and adjacent poles have different polarity, each of said four coil parts providing a symmetrical field distribution in reference to the two axes which are 45* from the vertical and horizontal axes and intersect at right angles at the center of the deflection yoke.
 6. A deflection yoke according to claim 5 wherein four coil parts are toroidally wound around the ring-shaped core and two of the four coils are opposed on the vertical axis and other two are opposed on the horizontal axis.
 7. A deflection yoke according to claim 5 wherein four flat-shaped coil parts are provided at the ring-shaped core and are arranged so that they are positioned on two axes deflected by 45* from the vertical and horizontal axes which intersect at a right angle. 